Yes, but not on my van.
About 23 years ago, I bought a 1970 VW Beetle that the owner thought had a burnt valve.
When I got it home we popped the valve covers to find something very similar to what you've posted.
However, instead of the rocker being off the valve-stem, it was off of the push-rod.
I put it back like it was supposed to be, made the proper valve lash adjustment, and drove it for two years with no other issues until I pulled the engine apart to soup it up.
Check to see if the rocker nut backed off...
Check to see if the push-rod's bent...
Check to see if there's damage to the valve stem...
The slack came from somewhere, if nothing's obvious, it's possible that you've had a lifter failure, a valve spring failure, or that cam lobe has an issue. | 
